The techniques of fluidizing a particulate material are well known and are advantageously applicable in boilers or more generally in enclosures for exothermic reactions. In this particular application, the heat released by the reaction or the combustion needs to be extracted as effectively as possible. Such heat extraction is generally performed by means of a heat exchanger, generally comprising a bundle of tubes having water under pressure flowing therein, with the water being turned into steam, with the heat exchanger extending over the walls of the hearth and/or through the fluidized bed itself in privileged zones thereof where corrosion and erosion conditions are judged to be relatively small. These tubes are thus situated over the fluidization grid which, in conventional manner, includes upwardly diverging openings through which the fluidizing gas (generally air) is blown, and further openings may be provided for admitting additional air. These further openings are placed at the top of small flues interposed between the diverging openings and the air which they deliver is directed generally horizontally or downwardly by means of deflectors or by virtue of special dispositions of the distribution orifices.
Fluidization grids are also known which include cooling tubes in contact with the fluidized bed. However, such grids are generally plane and do not have the advantages of grids having divergent gas-blowing openings.
The present invention seeks to provide a considerable improvement to this type of fluidized bed reactor, having a cooled grid in which the gas-blowing openings are divergent, thereby making control of the temperature within the fluidized bed easier than is possible using heat exchangers placed in the bed and which therefore wear very rapidly, the invention also makes it possible to heat the additional fluidizing air while simultaneously cooling the grid, thereby increasing its length of life.